Self-balancing potentiometer mechanism



July 8, 1947.

W. P. WILLS SELF-BALANCING POTENTIOIIETER MECHANISM 12 Sheets-Sheet 1 Filed Dec. 1, 1941 Onto Linc-Y July 8, 1947. w. P. WILLS SELF-BALANCING POTENTIOHETER MECHANISM Filed Dec. 1, 1941 12 Sheets-Sheet 2 RG3. smu. t ne me use "A FIGZ. mo

UPPLY VOLTAGE "Bill,

UTOR DRNE runes-emu VOLTAGE VOLTAGE noun mollmmo' Y WALTER P. WILLS y 8, 1947. w. P. WILLS 2,423,540

SELF-BALANCING POTENTIOMETER MECHANISM Filed Dec. 1, 1941 12 Sheets-Sheet 3 INVENTOR.

July 8, 1947.

W. P. WILLS SELF-BALANCING POTENTIOMETER MECHANISM Filed Dec. 1, 1941 FIG. IO.

12 Sheets-Sheet 4 INVENT OR.

WALTER R WILLS ORNEY y 1947- w. P. WILLS 2,423,540

SELF-BALANCING POTENTIOMETER MECHANISM Filed Dec. 1, 1941 12 Sheets-Sheet 5 FlG.ll.

INVENTDR.

WALTER P WILLS ly 1947- w. P. wiLLs 2,423,540

SELF-BALANCING POTENTIOMETER MECHANISM Filed Dec. 1, 1941 12 Sheets-Sheet 6 FIG. 13. 478

INVENTOR. WALTER P. WILLS AT RNEY July 8, 1947. P, MLLS 2,423,540

SELFBALANCING POTENTIOMETER MECHANI SM Filed Dec. 1, 1941 12 Sheets-Sheet 7 FIG. l4.

I ah A INVENT OR. WALTER P. WILLS AT RNEY y w. P. WILLS 2,423,540

SELF-BALANCING POTENTIOIETER MECHANISM Filed Dec. 1, 1941 l2 Sheets-Sheet 8 43 FIGJS. 533 5 O D 529 5 o 0 51%| F IGZI. 55! 492 480 FIG.I8.

4 46 48 FIGIS.

INVENTOR. 495 WALTER P. WILLS 3974971717 ?2 BY M 48 49 T TURN S.

July 8, '1947. w. P. WILLS SELF-BALANCING POTENTIOMETER MECHANISM Filed Dec. 1, 1941 12 Sheets-Sheet 10 I UIIIIIIIIIII/IIIIIIIIIIII mvsmon WALTER P. wuu s E :2 A'TRNEY y w. P. WILLS 2,423,540

SELF-BALANCING POTENTIOHETER MECHANISM Filed Dec. 1, 1941 12 Sheets-Sheet l1 4321 8'5 FIG.34.

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INVEN'IDR.

WALTER R WILLS Filed Dec. 1, 1941 12 Sheets-Sheet l2 FIG3T FIG.38.

INVENTOR. WALTER R WILLS RNEY Patented July 8, 1947 SELF-BALANCING POTENTIOMETER 1 MECHANISM Walter P. Wills, Philadelphia, Pa., assignor to The Brown Instrument Company, Philadelphia, Pa., a corporation of Pennsylvania Application December 1, 1941, Serial No. 421,173

52 Claims. 1

This invention relates to a measuring and/or control apparatus for measuring and/or controlling the value of a condition.

More specifically, this invention is directed to a self-balancing potentiometer measuring apparatus, wherein the unbalanced D. C. current of the potentiometer is transformed into a pulsating current of one phase or of opposite phase depending upon the sense of unbalance of the potentiometer circuit, wherein the pulsating current is amplified to operate a reversible electric motor in one direction or the other, wherein operation of the reversible electric motor rebalances the potentiometer circuit and wherein the electrio motor operates indicating, recording and/or control mechanism. The potentiometer apparatus may measure a condition and operate control mechanism to regulate that condition or some other condition.

A prime object of this invention is to provide a new and novel measuring and/or control apparatus for accomplishing in an improved manner measuring, indicating, recording and control functions. 33! reason of thi invention improved results in measuring, indicating, recording and control are obtained which cannot be obtained by present known measuring and/or control devices. by the coaction of new and novel features which provide more accurate and faster measuring, indicating, recording and controlling operations together with long life of the apparatus. These new and novel features and the coaction thereof also form objects of this invention.

More particularly, an object of this invention is to provide a. new and improved potentiometer circuit network and associated parts to obtain more accurate measurements.

Another object of this invention is to provide a new and novel means for transforming the D. C. current of the potentiometer circuit network into a pulsating current of one phase or of opposite phase depending upon the sense of unbalance of the potentiometer circuit network.

A further object of this invention is to provide new and novel means for amplifying the pulsating current and reversibly driving a reversible rotatable electric motor for performin indicating, recording and/or control functions.

Another object is to provide new and novel means for operating and preventing overtravel of a reversible electric motor.

Still another object of this invention isto properly construct and correlate the various parts of the measuring and/or control apparatu and to These improved results are accomplished associate therewith new and novel means for reducing to a minimum the eflects oi extraneous circulating or stray currents which would tend to hamper the satisfactory operation of the appa-.

ratus.

A rurther object of this invention is to provide a new and novel means for checking, calibrating and adjusting the sensitivity, zero position, operating range and other characteristics of the measuring and/or control apparatus.

In the case of a control apparatus, another object of this invention is to provide new and novel means to driv the control mechanism thereof to a safe position in case of failure of the measuring apparatus whereby safe control is provided.

Other objects and advantages will become apparent to those skilled in the art upon reference to the accompanying specification, claims and drawing in which:

Fig. 1 is a schematic illustration of the measuring and control apparatus;

Figs. 2, 3, 4, 5 and 6 are curves showing vol-tages and currents present in different parts oi the measuring and control apparatus under various operating conditions;

Fig. 7 is a front elevational view 01' the instrument with the door closed;

Fig. 8 is a side elevational view of the instrument with parts of the instrument case shown in section;

Fig. 9 is a front elevational view of the instrument with the door swung open;

.Fig. 10 is a front elevational view of the in strument with parts broken away and with the door, pointer and chart plate removed showing line |4 u of Figs. 10 and 11.

Fig. 15 is an elevational view of the slide-wire assembly with the cover thereof removed:

Fig. 16 is a vertical sectional view of the slidewire assembly;

Fig. 16A is a detail sectional view of the cable drum of the slide-wire assembly taken substantially along the line ISA-l 6A of Fig. 16;

1'? is a bottom view of the slide-wire assembiy with the cover thereof removed;

Figs. 18, 19, and 21 are diagrammatic views showing the manner in which the slide-wire and collector are secured to the slide-wire assembly;

Fig. 22 is a schematic view of the input transformer;

Figs. 23 and 24 are elevational views of the vibrator;

Fig. 25 is a sectional view taken substantially on the line 25-25 of Figs. 11 and 12;

Fig. 26 is a horizontal plan view looking down On Figs. 11, 12 and 25;

Fig. 27 is a detail sectional view of the cold junction housing;

Figs. 28 and 29 are elevational views of the clamping means for securing the instrument to a panel;

Fig. 30 is a. front view of the latch for the swinging frame;

Fig. 31 is a horizontal sectional view of the latch for the swinging frame;

Figs. 32 and 33 are elevational views of the stop for the door;

Fig. 34 is a schematic illustration of a measuring and control apparatus similar to that of Fig. 1 but including additional features;

Figs. 35 and 36 are schematic illustrations of alternative forms of thermocouples which can be used with measuring and control apparatus of Fig. 34; and

Figs. 37 and 38 are schematic illustrations of the measuring and control apparatus applied to D. C. and A. 0. bridge systems.

Referring now to Fig. 1 there is schematically illustrated the measuring and control apparatus in the form of a self-balancing potentiometer controller for measuring, indicating, recording and controlling the value of a condition,- for example, the temperature value in a furnace II. The furnace l0 may be heated by a burner ll controlled by a pneumatic valve 12.

A thermocouple l3 responsive to the temperature within the furnace ill operates in conjunction with a. potentiometer network generally designated at, 15 and a slide-wire assembly generally designated at iii to form a self-balancing potentiometer system. Upon a change in temperature within the furnace I!) an unbalanced D. C.

current in one direction or the opposite direction r is produced depending upon the sense of unbalance of the potentiometer system. This D. C. current of one direction or of opposite direction is supplied to an amplifier unit 11 having a vibrator or equivalent device generally designated at l8, a

transformer generally designated at l3, amplifying vacuum tubes generally designated at 20 and 2|, and motor drive vacuum tubes generally designated at 22 and 23.

The direct current supplied to the amplifier 11 is converted by the vibrator 18 into a pulsating current of one phase or of opposite phase depending upon the sense of unbalance of the potentiometer system. This pulsating current is detected and amplified by the transformer 13 and is further amplified by the amplifying vacuum tubes 20 and 2|. The output from the amplifying vacuum tubes 20 and 2| is supplied to the motor drive vacuum tubes 22 and 23 for operating in one direction or another a reversible electric motor generally designated at 24. The reversible motor 24 operates a drive mechanism generally designated at 25. The drive mechanism operates the slide-wire assembly 16 to rebalance the potentiometer system and also operates indicating 4 and recording means for indicating and recording the temperature conditions existing within the furnace Ill. The drive mechanism 23 also operates a controller generally designated at 24 which may take the form of a pneumatic controller such as illustrated in the C. B. Moore Patent 2,125,081, granted July 26, 1938. This controller 26 operates to control the valve l2 to maintain the temperature within the furnace III at the desired normal value.

The potentiometer network I! may comprise .izhree resistances 30, II, and 32 connected in series. of manganin wire having substantially no temperature resistance coefficient and are used for calibration purposes. Connected in parallel with the resistances 30, 3i and 32 are a battery 33 which may be in the form of a dry cell and a dual vernier rheostat comprising resistances 34 and II and electrically connected sliding contacts ll and 31 associated respectively with the resistances 34 and 35. The dual vernier rheostat may be operated by a knob 38, there being a direct mechanical connection between the knob and the contact 35 and a lost motion connection between the knob and the contact 31. Upon initial movement of the knob the contact 33 is first moved and then the contact 31 is moved thereby providing vernier adjustment. The battery 33 and the dual vernier rlieostat are connected in series with respect to each other. The resistances 34 and 35 of the vernier rheostat may be made of copper or other material having a positive temperature resistance coefficient for compensating for temperature changes in the battery 33. Also connected in parallel with the resistances 30, II and 32 at. two resistances 39 and 40 which in turn are connected in series with respect to each other. The resistance 39 is preferably made of copper or other material having a positive temperature resistance coefficient and the resistance 40 may be formed of manganin having substantially no temperature resistance coefficient. The resistance 39 operates to compensate for temperature changes at the cold Junction of the thermocouple, it being so connected into the network that the voltage drop across it is added to the thermocouple E. M. F. The resistance 43 is utilized primarily for standardizing purposes and has a resistance value such that the voltage drop across it is equal to the voltage produced by the standard cell.

Slide-wire assembly Hi may include a cable drum 43 suitably rotated by the drive mechanism 23 through a cable 44. The term cable is used in a generic sense to cover similar arrangements such as wire, violin string, rope, cable, etc. The cable drum 43 is provided with an adjustment for adjusting the diameter or circumference thereof. The cable drum 43 operates through an adjustable connection to move a contactor 43. The contactor 45 engages a slidewire in the form of a resistance wire 48 which is space wound around an insulated core 41. Preferably the slide-wire 43 and the core 41 are formed of the same material such as manganin wire having substantially no temperature resistance coefficient. The contactor 45 also engages a collector bar in the form of a resistance wire 43 space wound around a core 49 and completely electrically conductive with respect thereto throughout their lengths. Preferably, the resistance wire 43 and the core 49 are formed of the same material, such as manganin wire, as the resistance wire 44 and core 41. The slide-wire These resistances are preferably formed resistance wire 46 and the core 41 being connected together. Connected across the slide-wire 46 and core 41, that is in shunt with the slidewire 46, is a resistance 54 also formed of manganin wire. The terminals 52 and 53 are connected to the ends of the resistance element .3I of the potentiometer network I5 and thereby the slide-wire resistance 46, the resistance 54 and the resistance 3| are all connected in parallel.

A three-position switch generally designated at 51 is utilized for performing three functions, namely, for connecting the potentiometer circuit network for normal operation (run), forchecking the system for errors (check), and for standard-. izing the potentiometer system (standardize). schematically, this switch 51 may comprise switch arms 58, 59, 66 and 6| electrically insulated from each other and all operated by a common operating member 62. The switch arms 56 and 59 carry contacts 63 and 64, respectively. The switch arm 66 carries contacts 65 and 66 and the switch arm 6| carries contact61. Contact 63 engages a contact 66 when the switch 51 is moved to the standardize position and engages a contact 69 when the switch is moved to the check position. When the switch 51 is in the run position the contact 63 does not engage either contact 66 or 69. When the switch 51 is in run position the contact 64 engages both contacts 16a and Ila. When the switch is moved to the standardize position contact 64 disengages contact Ila and when the switch 51 is moved to check position the contact 64 disengages contact 16a. Contact 65 engages contact 14a when the switch 51 is in run position. When the switch 51 is moved to standardize position contact 65 disengages contact 14a and contacts 65 and 66 engage contacts 12a and 13a, respectively. When the switch 51 is movedto check position contact 65 remains in engagement with contact 14d and contact 66 engages contact a. When the switch 51 is in either the run position or the check position contact 61 does not engage contact 16a. Contact-61 engages contact 16a only when the switch 51 is moved to standardize position.

schematically the transformer I9 comprises a primary winding formed by two windings 16 and 1|. The adjacent ends of the winding 16 and 1| are connected together and to a terminal 12 and the other ends of the primary windings 16 and 1| are connected to terminals 13 and 14, respectively. The primary windings 16 and 1| are wound around a core structure 15 and also wound around the core structure is a secondary winding 16 connected between terminal 11 and 16. A shield 19 is provided between the primary windings I6 and 1| and the secondary winding 16. The windings and core structure of the transformer are housed in a casing 66. The core structure 15, the shield 19 and the casing 66 are connected to a terminal 6| which is in turn connected to ground.

schematically, the vibrator I8 may comprise a vibrating reed v63 for operating a contact 64 with respect to contacts 65 and 66. The contact 64 is connected to a terminal 61 and the contacts 65 and 66 are connected to terminals 66 and 69, respectively. The vibrating reed 63 is vibrated under the influence of a. winding 96 connected between two terminals 9| and 92 and supplied with alternating current. A permanent magnet 93 associated with the vibrating reed is utilized for polarizing and synchronizing purposes. The mountin structure for the vibrating reed, contacts, operating winding and permanent magnet is connectedt a terminal 94 which in turn is connected to ground A suitable housingn95. enclosesthe mechanismmf the vibrator I8.

For purposes of illustration it is assumed that the thermocouple I3 is an iron-constantan thermocouple. The lead I66 extending from the iron element of the thermocouple to the positive cold junction terminal I6I is an iron lead. The lead I62 leadin from the constantan element of the thermocouple to the negative cold junction terminal I63 is made of constantan. The hot Junction of the thermocouple is, therefore, in the furnace I6 and the cold junction is at the cold junction terminals IN and I63. The positive cold junction terminal IN is connected to the contacts 16a and 151101 the switch 51 and is also connected to a positive checking terminal I64. The negative cold Junction terminal I63 is connected to a negative checking terminal I65 which in turn is connected by a Jumper I66 to a negative checking terminal I61. The positive checking terminal I64 is connected to the terminal 12 at the junction of the two primary windings 16 and 1| of the transformer I9 and the opposite ends of these primary winding 16 and 1| are connected, respectively, through terminals 13 and 14 and terminals 69 and 66 to the contacts 66 and 65 of the vibrator I6, The vibrator contact 64 is connected through terminal 61 and the switch arm 66 to contacts 65 and 66. The contact 14a which normally engages the contact 65 is connected to the terminal 5| and hence to the collector bar 46 of the slide-wire assembly I6. Accordingly the positive lead of the thermocouple is connected through the transformer and the vibrator to the collector bar 46 of the slidewire assembly I6 when the switch 51 is in the run position. The negative checking terminal I61 is connected through a resistance I 66 to the contact 16a of the switch 51 and when the switch 51 is in the run position contact 16a is connected through contacts 64 and 1Ia to a point I69 located between the compensating resistance element 39 and the resistance element 46 of the potentiometer network I5.

The battery 33 operates to produce an E. M. F. between the contactor 45 of the slide-wire assembly I6 and the point I69 in the potentiometer network I5 which'oppose the E. M. F. produced by the thermocouple I3 and the value of the battery E. M. F. produced between these points is regulated by the position of the contactor 45 along the slide-wire resistance 46. As the contactor 45 of the slide-wire assembly is moved in a clockwise direction the battery E. M. F. between the contactor 45 and the point I69 is increased and as the contactor 45 is moved in a counter-clockwise direction the battery E. M. F. is decreased. When the potentiometer system is balanced the E. M. F. produced by the battery between the contactor 45 and the point I69 is equal and opposite to the E. M. F. produced by the thermocouple I3 and, therefore, no current flows in the thermocouple circuit.

Upon an increase in temperature within the furnace III the E. M. F. produced by thermocouple I 3 becomes greater than the E. M. F. produced by the battery 33 between the contactor 45 and the point I09 which causes a D. C. current flow from the positive cold junction terminal IOI through the checking terminal I04 to the terminal 12 of the transformer I9 and thence through the primary winding 10, terminals 13 and 99 and contacts 86 and 84 to the terminal 81 of the vibrator I8 or through the primary winding 1I, terminals 14 and 88 and contacts 85 and 84 to the terminal 81. From the terminal 81 the D. C. current flows through contacts 65 and 14a of the switch 51, terminal 5| of the slide-wire assembly, collector bar 48, contactor 45, slide-wire resistance 46, potentiometer network I5 to I09, and then through contacts lid, 64 and 10a of the switch 51, resistance I08, checking terminal I01, jumper I06, and checkin terminal I05 to the negative cold junction terminal I03. This produces a D. C. current in one direction which is detected and amplified by the amplifier I1 to operate the reversible motor 24 which through the drive mechanism 25, cable 44 and cable drum 43 moves the contact 45 in a clockwise direction to increase the battery E. M. F. between the contactor 45 and the point I09 to oppose equally the E. M. F. developed in the thermocouple. When this occurs the flow of D. C. current is stopped and the potentiometer system becomes balanced.

Upon a decrease in temperature within the furnace I the E. M. F. produced by the thermocouple I3 becomes less than the E. M. F. produced between the contactor 45 and the point I09 by the battery 33. This causes a D. C. current to flow in the opposite direction from the contactor 45 through collector bar 48, terminal 5| of the slide-wire assembly I6, contacts 14a and 65 of the switch 51, terminal 81 of the vibrator I8 and then through contacts 84 and 96, terminals 89 and 13 and primary winding 10 to the terminal 12 of the transformer I9 or through contacts 84 and 85, terminals 88 and 14 and primary winding 1! to the terminal 12. This D. C. current then flowsfrom the terminal 12 through positive checking terminal I04, positive cold junction terminal IOI, thermocouple I3, negative cold junction terminal I03, negative checking terminal I05, jumper I06, negative checking terminal I01, resistance I08, contacts 10a, 64 and Ho of the switch 51 and point I09 through the potentiometer network I5 to the contactor 45 of the slidewire assembly I6. This flow of D. C. current is detected and amplified by the amplifier I1 to operate the reversible motor 24 in the opposite direction which in turn operates through the drive mechanism 25, cable 44 and cable drum 43 to move the contactor 45 of the slide-wire assembly I6 in a. counter-clockwise direction. This decreases the E. M, F produced by the battery 33 between the contactor 45 and the point I09 to a value which equally opposes the E. M. F. produced by the thermocouple I3 whereupon the potentiometer system again becomes balanced and no D. C. current flows.

Accordingly upon a change in temperature within the furnace I0 an unbalanced D. C. current is caused to flow in one direction or the other depending upon whether the furnace temperature increases or decreases. This unbalanced D. C. current flow is detected and amplified by the amplifier I1 to operate the reversible motor 24 which in turn operates through the drive mechanism 25 and the slide-wire assembly I6 to reduce the unbalanced D. C. current flow to zero. The positions of the contactor 45 of the slide-wire assembly I6 and of the indicating, recording and controlling parts mechanically coupled thereto represent the actual temperature value existing within the furnace I0.

Changes in temperature at the cold junction terminals I M and I03 would normally tend to render the system inaccurate and in order to avoid this difllculty the cold junction compensation resistance 39 is included in the potentiometer network I5, this cold junction compensating resistance 39 being located in close proximity to the cold junction terminals IOI and I03, so as to be similarly affected by temperature conditions at this point. The cold junction compensation resistance element 39 is preferably made of copper having an accurate and uniform temperature resistance coeillcient. This resistance element 39 is so connected into the potentiometer network that the voltage drop across it .is added to the thermocouple E. M. F. whereby changes in temperature at the cold junction terminals III and I03 are entirely compensated for. A thermometer I I0 may be utilized for measuring the actual temperature at the cold junction terminals III and I03 to aid in the calibration of the potentiometer system.

The slide-wire assembly I6 in addition to including the slide-wire 46 also includes the shunt resistance 54. During the manufacture of the slide-wire assembly the shunt resistance 54 is so chosen with respect to the slide-wire resistance 46 that the total resistance of the slide-wire assembly is a predetermined value within very close limits. This resistance value is the same for all slide-wire assemblies regardless of the rest of the potentiometer system. As a consequence the r same slide-wire assembly I6 may be interchangeably utilized for different ranges and may also be interchangeably utilized in conjunction with various types of thermocouples, such as iron-constantan, copper-constantan and rare metal couples. All that is'necessary to adapt the potentiometer system for use in various ranges and for use with various types of thermocouples is to change the calibrating resistances 30, 3I and 32.

In prior art potentiometer systems it has been necessary to closely calibrate the calibrating resistances 30, 3| and 32 and the cold junction resistance 39 to cause the instrument to operate accurately. Any small inaccuracy in the values of the resistance elements 30, 32 and 39 would cause the zero position of the instrument to vary.

According to this invention an improved arrangement is provided. Here the resistances 30, 32 and 39 need only b constructed within reasonable limits instead of within the precise accurate limits which were previously required. To accurately align the zero position of the instrument, all that is necessary is to adjust the adjustable connection between the cable drum 43 and the contactor 45. This adjustable connection, therefore, provides an improved arrang ment for obtaining zero adjustment. Any sma; inaccuracy in the value of the resistance elemen 3I would normally cause th range of the instrument to vary. According to this invention the resistance element 3| need only be constructed within reasonable limits instead of within precise limits and accurate adjustment of the range may be obtained by adjusting the diameter or circumference of the cabl drum 43. This azijustment, therefore, provides an improved arrangement for obtaining range adjustment.

By having the resistances 3| and 54 connected in shunt with the slide-wire resistance 46, the

eifects of variations caused by variations in slideguano wire construction or wear of the slide-wire is greatly decreased. According to the usual practic in'the potentiometer art th impedance of the galvanometer or in this case the impedance of the input transformer I9 would be made substantially equal tothe resistance of the rest of the potentiometer circuit including the thermocouple I3 and extension leads, slide-wire assembly I6 and potentiometer network I5. In an unbalanced potentiometer circuit any change in resistance in the remainder of the potentiometer circuit would materially affect the sensitivity of the galvanometer or the secondary voltage produced by the input transformer. According to this invention the impedance of each half of the primary winding of the input transformer I9 at the frequency of conversion is made materially higher than the resistance of the remainder of the potentiometer circuit with the result that large changes in the resistance of the remainder of the potentiometer circuit have practically no effect on the secondary voltage produced by the input transformer I9. In a practical and satisfactory embodiment, it is found that the system is substantially unafiected by changes in resist- 1 ance when the impedance of each half of the primary of the transformer at the frequency of conversion is from to times the resistance of the rest of the potentiometer circuit. By reason of this selection of the input transformer impedance, changes" in thermocouple lead wire lengths, changes in contact resistance of the slide-wire assembly and the vibrator contacts, different selections of thermocouple materials and extension leads and the introduction into the potentiometer circuit ofadditional resistance has practically no effect on the over-all sensi tivity or operation of the complete system.

The resistance I08 connected to the negative ch'ecking terminal I01 and the condenser III connected across the thermocouple, are utilized for damping or anti-hunting operation and also for stray reduction purposes as will be pointed out more fully hereafter.

Power is supplied to the amplifier I1, the reversible motor 24 and a chart drive motor I2I| by means of line wires H5 and H6 leading from some source of alternating potential, not shown. For purposes of illustration it is assumed that the alternating voltage supplied by the line wires I I5 and H6 is 60 cycle. A double pole singl throw switch II1 connects conductors H8 and H9 to the power lines H5 and H6. The chart, drive motor I20 may be a synchronous motor including a winding I2I connected across the conductors H8 and H9, the winding I2I cooperating with a core structur I22 for rotating an armature I23 in a synchronous fashion; As will be pointed out more fully hereafter the armature I22 operates the chart drive mechanism.

10 throw switch It! and a fuse Ill. The purpose of the switch I33 is to adapt the transformer I2l for operation with either 110 voltage supply or 125 voltage supply. If the voltage supply is 125 volts the single pole double throw switch IN is in the position shown but if 110 volts are supplied the switch is thrown to the opposite position. The purpose of the switch I22 and the tap I22 is to maintain the number of volts per turn in the primary winding substantially constant regardless of whether 110 volts or 125 volts is supplied to the transformer.

The secondary winding H1 is utilized for supplying alternating current to the operating winding 90 of the vibrator I8 and to the filaments or heaters of the vacuum tubes 20, 2|, 22 and 28. The connections are designated 1: and Y. The secondary winding I21 is preferably grounded at mid voltage to maintain the filaments or heaters at ground potential.

The vibrator I8 is essentially a polarized switching mechanism, the operating winding 90 and the permanent magnet 93 cooperating to viand ISI. A shield I35 for stray elimination pura poses is located between the primary and secondary windings and this shield along with the core and cover of the transformer are connected to ground. The secondary windings I30 and Ill may be a single winding provided with a mid-tap I29. The primary winding I26 is provided with a tap I32 near one end thereof. The opposite end of theprimary winding I26 is connected to the conductor H9 and either the other end of the primary winding or the tap I32 is connected to the conductor H8 through a single pole double brate the vibrating reed 63 at 60 cycles per second in synchronism with the 60 cycle alternating voltage supply. For purpose of illustration it is assumed that the contact 86 is engaged by the contact 84 during the first half cycle of the alternating voltage supply when the voltage is positive and that the contact 85 is engaged by the contact 84 during the second half cycle when the voltage of the alternating voltage supp y is negative. Accordingly the contacts 84 and 86 engage when the voltage of the alternating voltage supply is positive and the contacts 85 and 84 engage when the voltage of the alternating voltage supply is negative. When the vibrating reed 83 is stationary in the mid position both contacts 85 and 86 are engaged by the contact 84 so that when the vibrating reed is operated the circuit through the primaries 10 and H of the transformer I9 is never entirely interrupted.

When the potentiometer system is balanced no current flows in the potentiometer system and, therefore, operation of the vibrator I8 has no effeet. When the temperature of the furnace III increases the unbalanced D. C. current in the potentiometer system flows in a direction from the transformer I9 to the vibrator I8 and vice versa, when the temperature within the furnace III decreases the unbalanced D. C. current in the potentiometer system flows from the vibrator I8 to the transformer I9. I

Referring to Figs. 2 to 6 curve A represents the phase of the supply voltage and it remains substantially the same at all times. Curve B of Fig. 2 represents the potential of the terminal 11 when the potentiometer is balanced, it being substantially constant. Upon a temperature increase the potential of the terminal 11 increases and decreases in phase with the supply voltage as illustrated by curves C of Figs. 3 and 4. It will be noted that the amplitude of curve C varies in accordance with the amount of potentiometer unbalance, it being greater for a larg unbalance. Curves D of Figs. 5 and 6 represent the potential of the terminal 11 when the temperature decreases'and here it is seen that the potential increases and decreases degrees out of phase with the supply voltage. Here again the amplitude of curve D varies with the amount of P tentiometer unbalance. While the various wave forms are not necessarily true sine waves, they are so shown in Figs. 2 to 6 to more clearly illustrate the principles of this invention.

Assume now that the temperature in the furnace I8 increases. During the first half cycle of the alternating voltage supplied by the supply lines H and M6 the unbalanced D. C. current fiows from the terminal 12 through primary winding 18, terminal 13, terminal 88 and contacts 86 and 84 to the terminal 81 producing a current fiow in the coil 18 to cause the terminal 12 to become more positive than the terminal 13. This current flow acts through the core structure 15 of the transformer I8 to induce a voltage in the secondary winding 16 causing the terminal 11 to be positive with respect to the terminal 18 during the first half cycle. During the second half cycle of the alternating voltage supplied by the lines H5 and H6 current fiows from the terminal 12 through the primary winding 1I, terminals 14 and 88 and contacts 85 and 84 to the terminal 81 of the vibrator I8. This causes a current fiow in the winding H to make the terminal 12 more positive than the terminal 14. This current flow in the primary winding 1 I acts through the core structure 16 to induce a. voltage in the secondary winding 16 to cause the terminal 11 to become negative with respect to the terminal 18. Accordingly, under these conditions an alternating voltage is produced across theterminals 11 and 18 which is in Phase with the alternating voltage of the supply lines H5 and I I6 as illustrated in Figs. 3 and 4 by curves C.

Assume now that the temperature in the fur nace I8 decreases. This causes an unbalanced D. C. current to flow in the opposite direction from the vibrator I8 to the transformer I8. During the first half cycle of voltage of the supply lines I I6 and I I6 current fiows from the terminal 81 of the vibrator I8 through contact 84 and 86, terminals 88 and 13 and primary winding 18 to the terminal 12. This causes a current to fiow through the primary winding 18 which makes the potential of the terminal 18 positive with respect to the terminal 12. This current fiow in the primary winding 18 operates in conjunction with the core structure 15 to induce a voltage in the secondary winding 16 to cause the potential of the terminal 11 to be negative with respect to the terminal 18 during the first half cycle. During the second half cycle of the alternating voltage supplied by the supply lines I I5 and I I6 current fiows from the terminal 81 of the vibrator I8 through contacts 84 and 85, terminals 88 and 14 and primary winding H to terminal 12. This causes a current flow in the primary winding H to cause the potential of the terminal 14 to be positi e with respect to the terminal 12. This current flow through the primary winding 1I operates through the core structure 15 to induce a voltage in the secondary winding 16 to cause the potential of the terminal 11 to be positive with respect to the terminal 18 during the second half cycle. Accordingly when the temperature of the furnace I8 decreases the unbalanced D. C. current fiow in the potentiometer system produces an alternating E. M. F. across the output terminals 11 and 18 of transformer I8 which is 180 out of phase with the alternating voltage of the supply lines I I5 and H6 as illustrated by curvesD of Figs. 5 and 6.

Summing up when the potentiometer system is balanced there is no current fiow through the primary winding 18 and H of the transformer I8 and hence the potential of the terminal 11 of the transformer I8 remains substantially constant. Upon an increase in furnace temperature the unbalanced D. C. current operates to cause 12 the potential of the terminal 11 to increase and decrease in phase with the supply voltage. Conversely upon a decrease in furnace temperature the unbalanced D. C. current fiow in the potentiometer circuit causes th potential of the terminal 11 to increase and decrease out of fiiase with the voltage of the supply lines I II and The amplifier I1 includes a continuous conductor I86 which is grounded at I31. The permanent magnet 83 and mounting structure of the vibrator I8 and the core structure 16, shield 18 and casing 88 of the transformer I8 are connected through this conductor I36 to ground. Likewise the secondary winding I21 of the transformer I25 is connected through the continuous conductor I86 to ground. The casing III of the amplifier I1 as well as the complete instrument casing is also connected to ground.

The vacuum tube 28 is a double triode tube of the indirect heated type and operates as two stages of amplification. The plate I48, cathode I42 and grid I43 are included in the first stage of amplification and the plate I44, the cathode I45 and the grid I46 are included in the second stage of amplification. The vacuum tube 2| is also a double triode tube, the plate I41, cathode I48 and grid I48 thereof acting as a third stage of amplification. The plate I58, cathode I5I and the grid I52 of the vacuum tube 2I act asarectifier for supplying D. C. current to the plate circuits of the three stages of amplification. In this connection the grid I52 is tied to the cathode I5I.

One end of the secondary winding I28 of the transformer I25 is connected to ground through the continuous conductor I36 and the other end is connected to the plate I58 of the rectifier. The plate I58, grid I52 and cathode I5I operate to produce a positive potential with respect to ground at the points I55, I56 and I51. The condenser I58 connected between the point I55 and the continuous conductor I36 to ground and the internal resistance of the tube 2I between the plate I58 and cathode I5I are utilized for filtering purposes to smooth out the ripple at the point I55. Likewise the resistance I58 and condenser I68 and the resistance I8I and condenser I62 operate to smooth out the ripples at the points I56 and I51. The points I55, I56 and I51 are, therefore, provided with one, two and three stages of filtering, respectively. Load resistances I65, I66 and I61 are connected into the plate circuits including the plates I48, I44 and I41, respectively.

The parallel arranged resistance I68 and condenser I68 located between the cathode I42 and the continuous conductor I36 to ground, are utilized for biasing the grid I43 negative with respect to the cathode I42. The resistance I 18 permits afiow of grid current between the grid I46 and the cathode I45 for limiting the extent to which the grid I46 may go positive with respect to the cathode I45. The resistance Ill and contactor I12 connected to the grid I48 perform a dual function, namely, limit the extent to "which the grid I48 may go positive with respect to the cathode I 48 and to vary the amount of signal impressed upon the grid I48 from the plate circuit of the second stage of amplification. The condenser I15 connected across the terminals 11 and 18 of the transformer I8 is provided for loading the secondary winding 16 of the transformer I8 to its ideal impedance at the line voltage frequency. The condenser I16 connected between the plate I48 of the first stage of amplification and the grid I48 01 the second stage of ampliflcation is provided for eliminating the D. C. component of the plate circuit voltage on the grid I48 and for impressing the alternating component of the voltage produced across resistor I85 to the grid I48. Likewise the condenser I11 connected between the plate I44 of the second stage or amplification and the grid I48 of the third stage of amplification is utilized for eliminating the D. C. component of the plate circuit voltage on the grid I48 and for impressing the alternating component of the voltage produced across resistor I88 to the grid I48.

The motor drive vacuum tubes 22 and 23 may also be double triode tubes. The vacuum tube 22 includes a plate I88, cathode I8I and grid I82 and also a plate I83, cathode I84 and grid I85. The vacuum tube 23 includes a plate I88, a cathode I81 and grid I88 and alsoa plate I89, cathode I98 and grid I9I. The plates I88 and I88 of the vacuum tubes 22 and 23 are connected to the left .end of the transformer secondary winding I38 and the plates I83 and I89 are connected to the right end of the transformer secondary winding I3I. The cathodes I8I, I84, I81 and P98 are connected together and through a biasing resistance I93 and the continuous conductor I38 to ground. The grids I82, I85, I88 and I! are connected together and are also connected through a condenser I94 to the plate I41 of the third stage of amplification. The condenser I94 is provided for eliminating the D. C. component of the plate circuit voltage on the grids I82, I85, I88 and I9I, and also for impressing the alternating component of the voltage produced across the resistance I81 to these grids. The resistances I95 and I98 and the portion of the transformer secondary winding I28 to the right of point I91 permit a flow of current between the grids I82, I85, I88 and I9l and the cathodes I8I, I84, I81 and I98, respectively, through the resistance I93 for limiting the extent to which the grids ray go positive with respect to their associated cathodes. The transformer secondary winding I28 and the resistance I 98 also provide a source of alternating voltage to be applied to the grids I82, I85, I88 and I9I for a purpose to be pointed out more fully hereafter. The signal from the third stage of amplification is impressed simultaneously and equally on all of the control grids of the two vacuum tubes 22 and 23. From the above connections it will be noted that the vacuum tubes 22 and 23 are connected in parallel for similar and simultaneous operation.

As illustrated, the motor 24 is a rotating field motor including a stator 288- with four pole pieces physically spaced 90 apart and a squirrel cage rotor 28I having interconnected conductor bars. Two of the opposite pole pieces are provided with series connected power windings 282 and 283 and the other two opposite pole pieces are provided with series connected control windings 284 and 285. When the voltage and current through the motor control windings lag the voltage and current, respectively, in the power windings the rotor 28I rotates in a clockwise direction. When the voltage and current in the control windings lead the voltage and current, respectively, in the power windings the rotor 28I rotates in a counter-clockwise direction. The motor 24 is preferably so constructed that the control windings 284 and 285 have a high impedance to match the plate impedance of the motor drive vacuum tubes 22 and 23 when the motor is operating and also that the power windings 282 and 283 have a high windings.

impedance in order to provide for eiiicient operation. Preferably, the control and power windings of the motor have a high ratio of inductive reactance to resistance, for example, from 6 to 1 to 8 to 1 at 60 cycles. This provides for maximum power during running with the least amount of heating and also provides a low impedance path in the control windings for anti-hunting purposes. In addition, because of this high ratio of inductive reactance to resistance in the power windings of the motor, the latter heats less when stalled than when running.

Electrical energy is supplied to the power windings 282 and 283 of the motor 24 from the conductor II8 through condenser 288, conductor 281, power windings 282 and 283, and conductor 288 back to the conductor H9. The condenser 288 is so selected with respect to the power windings .282 and 283 as to provide a substantially series resonant circuit when the rotor 28I is rotating at approximately full speed. Due to this series resonant circuit the total impedance of the power winding circuit is substantially equal to the resistance of the power windings 282 and 283 and since this resistance is relatively low, a large current flow through the power windings is made possible. This results in the production of maximum power and torque by the motor. Due to the series resonant circuit the current flow through the power windings 282 and 283 is in phase with the supply voltage. However, the voltage across the power windings 282 and 283 leads the current by substantially this being caused by the inductance of the windings. In Figs. 4 and 6 curve E shows the current flow in the motor power windings 282 and 283 to be substantially in phase with the supply voltage, and curve F shows the voltage across the power windings leading the current by substantially 90 when the rotor 28I is operating at approximately maximum speed. When the rotor 28I is operating at substantially maximum speed, the inductance of the power windings 282 and 283 is a maximum giving the above referred to series resonant condition. As the speed of rotation bf the rotor :28l falls off the inductance of the power windings 282 and 283 decreases thereby upsetting to some extent the resonant condition. This causes a slight phase shift in the current through the power windings and the voltage across the power windings, the voltage shifting slightly more than the current which reduces the power loss in the power windings. Further, the change from the resonant condition reduces the current flow through the power windings and due to the decrease in inductance the voltage across the power windings likewise decreases. This also reduces the power loss in the As a net result there is a substantial reduction in heating of the power windings when the rotor 28I mat a standstill as compared to a condition when the rotor is operating at substantially maximum speed. Curves F and E of Fig. 2 show the voltage across and the current through the power windings when the rotor is at a standstill and curves F and E of Figs. 3 and 5 show the same conditions when the rotor is operating at an intermediate speed. The amplitudes of the currents and voltages and the phase shifts are set forth in Figs. 2 to 6.

Power is supplied to the control windings 284 and 285 from the transformer secondary windings I38 and I3I through the plate circuits of the motor drive vacuum tubes 22 and 23, resistance I93, continuous conductor I38, conductor 289, control windings 284 and 285 and conrunning conditions of the motor.

ductor M to the center tap I29 of the transformer secondary windings I30 and I3I. A condenser 2II is connected in parallel with the control windings 204 and 205 and this condenser is so selected as to provide a parallel resonant circuit during both the stalled and This parallel resonant circuit presents a relatively high external impedance and a relatively low internal circuit impedance. The relatively high external impedance of this parallel resonant circuit matches the impedance of the plate circuits of the vacuum tubes 22 and '23 whereby the optimum conditions of operation are approached. The relatively low internal circuit impedance approximates the actual resistance of the control windings 204 and 205, and since this resistance is relatively low, the impedance of the internal circuit is relatively low.

During the first half cycle of the alternating supply voltage the plate I80 of the motor drive tube 22 is positive with respect to the center point I29 of the transformer I25, and during the second half cycle the plate I80 is negative with respect to the center point I20 as illustrated by the curves G in Figs. 2 to 6. During the first half cycle of the alternating voltage the plate I83 of the motor drive vacuum tube 22 becomes negative with respect to the center point I29 and during the second half cycle the plate I83 becomes positive with respect to the point i29, as illustrated by curves H of Figs. 2 to 6. The voltage of the plate I80 accordingly alternates in phase with the supply voltage and the voltage of the plate I83 alternates 180 degrees out of phase with the supply voltage. This relation always remains substantially the same.

On the assumption that the potentiometer system is balanced and, therefore, no unbalanced D. 0. current flows in the potentiometer system, and on the further assumption that the point of connection of resistors I95 and I96 is connected directly to the continuous conductor I38, the potential of th terminal ll of the transformer I9 remains substantially the same as the potential of the terminal I8 which is at ground potentials as illustrated by curve B of Fig. 2. The potential of the grids I43, I48 and I48 of the three stages of amplification remain substantially constant, and no signal is impressed upon the grids I82, I85, I88 and I9l of the motor driv vacuum tubes 22 and 23. Accordingly the potential of the grids of the motor drive vacuum tubes 22 and 23 remains substantially constant as illustrated by the curve I of Fig. 2.

Under these conditions current flows from the plate I80 to the cathod |8i when the plate is positive during the first half cycle, but current will not flow in this circuit during the second half cycle. During the first half cycle a pulse of current, therefore, passes from the plate I 80 to th cathode I8I as illustrated by curve J in Fig. 2. During the first half cycle when the plate I83 is negative no current will flow from the plate I83 to the cathode I84, but during the second half cycle when the plate I83 is positive current will flow from the plate I83 to the cathode I84 as illustrated by the curve K of Fig. 2. Since the plate I86, cathode I87 and grid I88 of the vacuum tube 23 are connected in parallel with the plate I80, cathode I8I and grid I82 of tube 22, and since the plat I89, cathode I90 and grid I9I of the vacuum tube 23 are connected in parallel with the plate I83, cathode I84 and grid I85 of tube 22, the vacuum tube 23 is connected in parallel with the vacuum tube 22 and operates in exactly th same manner as the vacuum tube 22.

Since the grids I82, I85, I88 and ISI are all connected together and since the potentials of these grids remain constant when the potentiometer circuit is balanced and the point of connection of resistors I and I 96 is connected to the conductor I30, the vacuum tubes 22 and 23 produce pulses of current of equal magnitude during each half cycle of th supply voltage as is illustrated by the curves J and K of Fig. 2 The plate circuit of the vacuum tubes 22 and 23 include the parallel resonant circuit formed by the motor control windings 204 and 205 and the condenser 2I I. The parallel resonant circuit offers a high external impedance which is substantially resistive in character, and accordingly a pulsating voltage drop in phase with the plate current is produced across the control windings 204 and 205 by the flow of the pulsating plate current through the parallel resonant circuit.

The pulsating voltage across the control windings 204 and 205 produces a current through these windings which includes a D. C. component and an alternating current having a fun damental frequency of cycles. Due to the inductance in the motor control windings 204 and 205 the current flow through the motor control windings is caused to lag the voltage across the windings by 90 degrees of this 120 cycle voltage or by 45 degrees of the 60 cycle supply voltage. The pulsating voltage across the motor control windings 204 and 205 is illustrated by the curve L of Fig. 2 and the alternating current flowing through the motor control windings 204 and 20-5 is illustrated by th curve M of Fig. 2. The condenser 2H connected in parallcl with the motor control windings operates to maintain the D. C. component of the current flow therethrough at a substantially steady value and also provides a low impedance path for the 120 cycle current flow in the motor control windings. Since the motor control windings have a low resistance the D. C. component of the current flowing through the motor control windings is relatively great while the amplitude of the alternating component of the current flowing through the motor control windings is relatively small because of the relatively high inductance of the motor control windings. The D. C. component of the current flowing through the motor control windings is illustrated by the curve N of Fig. 2. Due to the relatively high D. C. current flowing through the motor control windings 204 and 205 when the potentiometer system is balanced, the core structure tends to become saturated and the inductive reactanc of the motor control windings is relative small. The condenser 2I| is so selected with respect to this inductive reactance at 120 cycles that the condenser in parallel with the motor control windings forms a substantially parallel resonant circuit.

The relatively large D. C. current flowing through the motor control windings When the potentiometer system is balanced acts as a brake to prevent rotation of the rotor 20I. Rotation of the rotor causes the conductor bars thereof to cut flux produced by the D. C. current flowing through the motor control windings 204 and 205 and this produces a relatively heavy current in the rotor conductor bars which in turn quickly expends the force of rotation of the rotor. In other words, the developing of a relatively heavy 

